Is the European Circuit-Breaker Rating System Really More Conservative than the American? [includes discussion]

Abstract

use of more controlled kilovars at the receiving-end stations. It is not economically justifiable to transmit heavy amounts of reactive power over the lines. Instead, synchronous generators are installed in the receiving-end stations. A first unit of 75 megavolt-amperes is in operation and two more units are in order. The kilovar loading will decrease the required voltage range of the regulating transformers to some extent, but not very much. The range is determined by studying different operating conditions on the network analyzer. Today the voltage level is 400 kv both in power stations and receiving stations.The first two 400-kv series capacitors will be equipped with protective spark gaps without forced deionization. They will be set to ignite only for faults on the compensated line itself. They are guaranteed not to reignite after successful automatic reclosure of the line breakers with a dead interval of 0.2 second. For our third 400-kv series capacitor, planned to be placed in service in 1955, we shall probably use gaps with forced deionization since the gaps would otherwise have to be set at such a high value that the capacitors would be unduly expensive.As to the transformer bushings and the cable connections, it must be borne in mind that the design is chosen for transformers located in cells blasted in the rock.A transformer set consists of three very large single-phase units and a fourth as a standby. Each unit has its own cable. The exchanging of transformer and cable units can take place under voltage or, in some power stations, with disconnectors in about 10 minutes. In this case the economic gains from the design are so great that we have utilized it in spite of apprehensions of the kind referred to by Mr. Madill.W1'e hope that we have clarified our views on the points brought up in the discussions, but we shall be very glad to continue the Synopsis: A European power circuit breaker is given two interrupting ratings, one for the interruption of symmetrical currents only and one, usually about 25 per cent larger, for the interruption of asymmetrical currents only. In contrast, an American power circuit breaker is given a single rated interrupting current at rated voltage which applies to both symmetrical and asymmetrical currents, and which is numerically the same as the rated symmetrical current of a corresponding European breaker. The larger rated asymmetrical interrupting current of a European circuit breaker is not accompanied by a correspondingly larger rated making current and therefore can be used only when the current to be interrupted is increased without increasing the initial current. To use the full 25-per-cent extra, the relays and breaker must operate rapidly enough to separate the breaker contacts in about 2 cycles of a 60-cycle wave after the short circuit starts. If the relay time plus breaker opening time is 4 cycles or more, as is assumed for 8-cycle breakers in American-recommended application procedure, a typical circuit will not give an asymmetrical current which, at the time ...